Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting layer, which is a stack of a first light-emitting layer and a second light-emitting layer, is provided between an anode and a cathode. The first light-emitting layer is formed on the anode side and contains a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property. The second light-emitting layer contains a second light-emitting substance converting triplet excitation energy into light emission, the first organic compound, and a third organic compound having a hole-transport property. The second organic compound has a lower HOMO level than the third organic compound. The first light-emitting substance emits light with a wavelength shorter than that of light emitted from the second light-emitting substance. The first and the second organic compounds form an exciplex. The first and the third organic compounds form an exciplex.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement in which an organic compound capable of providing light emissionby application of an electric field is provided between a pair ofelectrodes, and also relates to a light-emitting device, an electronicdevice, and a lighting device including such a light-emitting element.

2. Description of the Related Art

Light-emitting elements including an organic compound as a luminousbody, which have features such as thinness, lightness, high-speedresponse, and DC driving at low voltage, are expected to be applied tonext-generation flat panel displays. In particular, display devices inwhich light-emitting elements are arranged in a matrix are considered tohave advantages of a wide viewing angle and high visibility overconventional liquid crystal display devices.

It is said that the light emission mechanism of a light-emitting elementis as follows: when a voltage is applied between a pair of electrodeswith an EL layer including a luminous body provided therebetween,electrons injected from the cathode and holes injected from the anoderecombine in the light emission center of the EL layer to form molecularexcitons, and energy is released and light is emitted when the molecularexcitons relax to the ground state. A singlet excited state and atriplet excited state are known as the excited states, and it is thoughtthat light emission can be obtained through either of the excitedstates.

In order to improve element characteristics of such light-emittingelements, improvement of an element structure, development of amaterial, and the like have been actively carried out (see, for example,Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2010-182699

SUMMARY OF THE INVENTION

However, it is said that the light extraction efficiency of alight-emitting element at present is approximately 20% to 30%. Evenconsidering light absorption by a reflective electrode and a transparentelectrode, the external quantum efficiency of a light-emitting elementincluding a phosphorescent compound has a limit of approximately 25% atmost.

In one embodiment of the present invention, a light-emitting elementwith high external quantum efficiency is provided. In another embodimentof the present invention, a light-emitting element having a longlifetime is provided.

One embodiment of the present invention is a light-emitting elementincluding a light-emitting layer between a pair of electrodes (anode andcathode). The light-emitting layer has a stacked-layer structureincluding a first light-emitting layer and a second light-emittinglayer. The first light-emitting layer is formed on the anode side andcontains at least a first light-emitting substance (guest material)converting triplet excitation energy into light emission, a firstorganic compound (host material) having an electron-transport property,and a second organic compound (assist material) having a hole-transportproperty. The second light-emitting layer contains at least a secondlight-emitting substance (guest material) converting triplet excitationenergy into light emission, the first organic compound (host material)having an electron-transport property, and a third organic compound(assist material) having a hole-transport property. In the firstlight-emitting layer, a combination of the first organic compound (hostmaterial) and the second organic compound (assist material) forms anexciplex, and in the second light-emitting layer, a combination of thefirst organic compound (host material) and the third organic compound(assist material) forms an exciplex.

Another embodiment of the present invention is a light-emitting elementincluding a light-emitting layer between an anode and a cathode, ahole-transport layer between the anode and the light-emitting layer, andan electron-transport layer between the cathode and the light-emittinglayer. The light-emitting layer is a stack of a first light-emittinglayer and a second light-emitting layer. The first light-emitting layeris formed in contact with the hole-transport layer and contains at leasta first light-emitting substance converting triplet excitation energyinto light emission, a first organic compound having anelectron-transport property, and a second organic compound having ahole-transport property. The second light-emitting layer is formed incontact with the electron-transport layer and contains at least a secondlight-emitting substance converting triplet excitation energy into lightemission, the first organic compound having an electron-transportproperty, and a third organic compound having a hole-transport property.In the first light-emitting layer, a combination of the first organiccompound (host material) and the second organic compound (assistmaterial) forms an exciplex, and in the second light-emitting layer, acombination of the first organic compound (host material) and the thirdorganic compound (assist material) forms an exciplex.

Note that in each of the above structures, the emission wavelength ofthe exciplex formed by the first organic compound (host material) andthe second organic compound (assist material) in the firstlight-emitting layer is located on the longer wavelength side withrespect to the emission wavelength (fluorescent wavelength) of each ofthe first and second organic compounds (host and assist materials).Therefore, by formation of the exciplex, the fluorescence spectrum ofthe first organic compound (host material) and the fluorescence spectrumof the second organic compound (assist material) can be converted intoan emission spectrum which is located on the longer wavelength side.Further, the emission wavelength of the exciplex formed by the firstorganic compound (host material) and the third organic compound (assistmaterial) in the second light-emitting layer is located on the longerwavelength side with respect to the emission wavelength (fluorescentwavelength) of each of the first and third organic compounds (host andassist materials). Therefore, by formation of the exciplex, thefluorescence spectrum of the first organic compound (host material) andthe fluorescence spectrum of the third organic compound (assistmaterial) can be converted into an emission spectrum which is located onthe longer wavelength side.

Note that in each of the above structures, in the first light-emittinglayer, an exciplex is formed from an anion of the first organic compoundand a cation of the second organic compound, and in the secondlight-emitting layer, an exciplex is formed from an anion of the firstorganic compound and a cation of the third organic compound.

In the above structures, the first light-emitting layer and the secondlight-emitting layer contain the same first organic compound (hostmaterial), and the highest occupied molecular orbital level (HOMO level)of the second organic compound (assist material) contained in the firstlight-emitting layer is lower than that of the third organic compound(assist material) contained in the second light-emitting layer.

In addition, in the above structures, the first light-emitting substancecontained in the first light-emitting layer is a substance which emitslight with a wavelength shorter than that of light emitted from thesecond light-emitting substance contained in the second light-emittinglayer.

In the above structures, the first light-emitting substance and thesecond light-emitting substance are light-emitting substances whichconvert triplet excitation energy into light emission, andphosphorescent compounds such as an organometallic complex, or amaterial emitting thermally activated delayed fluorescence, i.e., athermally activated delayed fluorescence (TADF) material, can be used.Further, the first organic compound is mainly an electron-transportmaterial with an electron mobility of 10⁻⁶ cm²/Vs or more, specifically,a π-electron deficient heteroaromatic compound. The second organiccompound and the third organic compound are mainly hole-transportmaterials with a hole mobility of 10⁻⁶ cm²/Vs or more, specifically,π-electron rich heteroaromatic compounds or aromatic amine compounds.

Further, the present invention includes, in its scope, electronicdevices and lighting devices including light-emitting devices, as wellas light-emitting devices including light-emitting elements. Thelight-emitting device in this specification refers to an image displaydevice and a light source (e.g., a lighting device). In addition, thelight-emitting device includes all the following modules: a module inwhich a connector, such as a flexible printed circuit (FPC) or a tapecarrier package (TCP), is attached to a light-emitting device; a modulein which a printed wiring board is provided at the end of a TCP; and amodule in which an integrated circuit (IC) is directly mounted on alight-emitting element by a chip-on-glass (COG) method.

Note that in a light-emitting element that is one embodiment of thepresent invention, an exciplex can be formed in each of the firstlight-emitting layer and the second light-emitting layer which areincluded in the light-emitting layer; thus, the light-emitting elementcan have high energy transfer efficiency and high external quantumefficiency.

Further, in a light-emitting element that is one embodiment of thepresent invention, owing to the above-described element structure, theexciplex formed in the first light-emitting layer has higher excitationenergy than the exciplex formed in the second light-emitting layer;therefore, when a substance emitting light with a wavelength shorterthan that of light emitted from the second light-emitting substance(guest material) which is contained in the second light-emitting layerand converts triplet excitation energy into light emission is used asthe first light-emitting substance (guest material) which is containedin the first light-emitting layer and converts triplet excitation energyinto light emission, the first light-emitting layer and the secondlight-emitting layer can provide light emission at the same time.Further, part of excitation energy of the exciplex formed in the firstlight-emitting layer which does not contribute to light emission can beutilized as excitation energy for the second light-emitting substance(guest material) which converts triplet excitation energy into lightemission in the second light-emitting layer. Accordingly, emissionefficiency in the light-emitting element can be further increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a concept of one embodiment of the presentinvention.

FIG. 2 illustrates a concept of one embodiment of the present invention.

FIG. 3 show calculation results according to one embodiment of thepresent invention.

FIGS. 4A1, 4A2, 4B1, 4B2, 4C1, and 4C2 show calculation resultsaccording to one embodiment of the present invention.

FIG. 5 illustrates a structure of a light-emitting element.

FIGS. 6A and 6B each illustrate a structure of a light-emitting element.

FIG. 7 illustrates a light-emitting device.

FIGS. 8A and 8B illustrate a light-emitting device.

FIGS. 9A to 9D illustrate electronic devices.

FIGS. 10A to 10C illustrate an electronic device.

FIG. 11 illustrates lighting devices.

FIG. 12 illustrates a light-emitting element.

FIG. 13 shows current density-luminance characteristics of alight-emitting element 1.

FIG. 14 shows voltage-luminance characteristics of a light-emittingelement 1.

FIG. 15 shows luminance-current efficiency characteristics of alight-emitting element 1.

FIG. 16 shows voltage-current characteristics of a light-emittingelement 1.

FIG. 17 shows an emission spectrum of a light-emitting element 1.

FIG. 18 shows emission spectra of substances used in a light-emittingelement 1.

FIG. 19 shows emission spectra of substances used in a light-emittingelement 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings. Note that the present invention is notlimited to the following description, and various changes andmodifications can be made without departing from the spirit and scope ofthe invention. Therefore, the present invention should not be construedas being limited to the description in the following embodiments.

(Elementary Process of Light Emission in Light-Emitting Element)

First, a description is given of general elementary processes of lightemission in a light-emitting element which uses a light-emittingsubstance converting triplet excitation energy into light emission(e.g., a phosphorescent compound or a thermally activated delayedfluorescence (TADF) material) as a guest material. Note that a moleculeproviding excitation energy is referred to as a host molecule, while amolecule receiving the excitation energy is referred to as a guestmolecule.

(1) The case where an electron and a hole recombine in a guest molecule,and the guest molecule is put in an excited state (direct recombinationprocess).

(1-1) When the excited state of the guest molecule is a triplet excitedstate, the guest molecule emits phosphorescence.

(1-2) When the excited state of the guest molecule is a singlet excitedstate, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state and emitsphosphorescence.

In other words, in the direct recombination process in (1), as long asthe efficiency of intersystem crossing and the phosphorescence quantumyield of the guest molecule are high, high emission efficiency can beobtained. Note that the T₁ level of the host molecule is preferablyhigher than the T₁ level of the guest molecule.

(2) The case where an electron and a hole recombine in a host moleculeand the host molecule is put in an excited state (energy transferprocess).

(2-1) When the excited state of the host molecule is a triplet excitedstate and the T₁ level of the host molecule is higher than the T₁ levelof the guest molecule, excitation energy is transferred from the hostmolecule to the guest molecule, and thus the guest molecule is put in atriplet excited state. The guest molecule in the triplet excited stateemits phosphorescence. Note that energy transfer from the T₁ level ofthe host molecule to a singlet excitation energy level (S₁ level) of theguest molecule is forbidden unless the host molecule emitsphosphorescence, and is unlikely to be a main energy transfer process;therefore, a description thereof is omitted here. In other words, energytransfer from the host molecule in the triplet excited state (3H*) tothe guest molecule in the triplet excited state (3G*) is important asrepresented by Formula (2-1) below (where 1G represents the singletground state of the guest molecule and 1H represents the singlet groundstate of the host molecule).3H*+1G→1H+3G*  (2-1)

(2-2) When the excited state of the host molecule is a singlet excitedstate and the S₁ level of the host molecule is higher than the S₁ leveland T₁ level of the guest molecule, excitation energy is transferredfrom the host molecule to the guest molecule, and thus, the guestmolecule is put in a singlet excited state or a triplet excited state.The guest molecule in the triplet excited state emits phosphorescence.In addition, the guest molecule in the singlet excited state undergoesintersystem crossing to a triplet excited state, and emitsphosphorescence.

In other words, there can be a process where energy is transferred fromthe host molecule in the singlet excited state (1H*) to the guestmolecule in the singlet excited state (1G*) and then the guest moleculeis put in the triplet excited state (3G*) by intersystem crossing, asrepresented by Formula (2-2A) below, and a process where energy isdirectly transferred from the host molecule in the singlet excited state(1H*) to the guest molecule in the triplet excited state (3G*), asrepresented by Formula (2-2B) below.1H*+1G→1H+1G*→(Intersystem crossing)→1H+3G*  (2-2A)1H*+1G→1H+3G*  (2-2B)

When all the energy transfer processes described above in (2) occurefficiently, both the triplet excitation energy and the singletexcitation energy of the host molecule are efficiently converted intothe triplet excited state (3G*) of the guest molecule. Thus,high-efficiency light emission is possible. In contrast, before theexcitation energy of the host molecule is transferred to the guestmolecule, when the host molecule itself is deactivated by emitting theexcitation energy as light or heat, the emission efficiency decreases.

Next, factors controlling the above-described processes ofintermolecular energy transfer between the host molecule and the guestmolecule are described. As mechanisms of the intermolecular energytransfer, the following two mechanisms are proposed.

One mechanism is Forster mechanism (dipole-dipole interaction) in whichenergy transfer does not require direct contact between molecules andenergy is transferred through a resonant phenomenon of dipolaroscillation between a host molecule and a guest molecule. By theresonant phenomenon of dipolar oscillation, the host molecule providesenergy to the guest molecule, and thus, the host molecule is put in aground state and the guest molecule is put in an excited state. Notethat the rate constant k_(h)*_(→g) of Forster mechanism is expressed byFormula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{k_{h^{*}\rightarrow g} = {\frac{9000\; c^{4}K^{2}\phi\mspace{11mu}\ln\mspace{11mu} 10}{128\;\pi^{5}n^{4}N\mspace{11mu}\tau\; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{dv}}}}} & (1)\end{matrix}$

In Formula (1), v denotes a frequency, f′_(h)(v) denotes a normalizedemission spectrum of a host molecule (a fluorescence spectrum in energytransfer from a singlet excited state, and a phosphorescence spectrum inenergy transfer from a triplet excited state), ε_(g)(v) denotes a molarabsorption coefficient of a guest molecule, N denotes Avogadro's number,n denotes a refractive index of a medium, R denotes an intermoleculardistance between the host molecule and the guest molecule, τ denotes ameasured lifetime of an excited state (fluorescence lifetime orphosphorescence lifetime), c denotes the speed of light, ϕ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host molecule and the guest molecule. Note that K²=⅔ inrandom orientation.

The other is Dexter mechanism (electron exchange interaction) in which ahost molecule and a guest molecule are close to a contact effectiverange where their orbitals overlap, and the host molecule in an excitedstate and the guest molecule in a ground state exchange their electrons,which leads to energy transfer. Note that the rate constant k_(h)*_(→g)of Dexter mechanism is expressed by Formula (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\;\pi}{h} \right)K^{\prime 2}{\exp\left( {- \frac{2\; R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{dv}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K′ denotes a constanthaving an energy dimension, v denotes a frequency, f′_(h)(v) denotes anormalized emission spectrum of a host molecule (a fluorescence spectrumin energy transfer from a singlet excited state, and a phosphorescencespectrum in energy transfer from a triplet excited state), ε′_(g)(v)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is thought to beexpressed by Formula (3). In the formula, k_(r) denotes a rate constantof a light-emission process (fluorescence in energy transfer from asinglet excited state, and phosphorescence in energy transfer from atriplet excited state) of a host molecule, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of a host molecule, and τ denotes a measured lifetime of anexcited state of a host molecule.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

According to Formula (3), it is found that the energy transferefficiency Φ_(ET) can be increased by increasing the rate constantk_(h)*_(→g) of energy transfer so that another competing rate constantk_(r)+k_(n)(=1/τ) becomes relatively small.

(Energy Transfer Efficiency in (2-1))

Here, the energy transfer process in (2-1) is considered first. SinceForster mechanism (Formula (1)) is forbidden in this case, only Dextermechanism (Formula (2)) should be considered. According to Formula (2),in order to increase the rate constant k_(h)*_(→g), it is preferablethat an emission spectrum of a host molecule (here, a phosphorescencespectrum because energy transfer from a triplet excited state isdiscussed) largely overlap with an absorption spectrum of a guestmolecule (absorption corresponding to direct transition from a singletground state to a triplet excited state).

In one embodiment of the present invention, a light-emitting substanceconverting triplet excitation energy into light emission (e.g., aphosphorescent compound or a thermally activated delayed fluorescence(TADF) material) is used as a guest material. In an absorption spectrumof the phosphorescent compound, absorption corresponding to directtransition from a singlet ground state to a triplet excited state isobserved in some cases, which is an absorption band on the longestwavelength side. In particular, light-emitting iridium complexes have abroad absorption band at around 500 nm to 600 nm as the absorption bandon the longest wavelength side (as a matter of fact, the broadabsorption band can be on a shorter or longer wavelength side dependingon emission wavelengths). This absorption band is mainly based on atriplet MLCT (metal to ligand charge transfer) transition. Note that itis considered that the absorption band also includes absorptions basedon a triplet π-π* transition and a singlet MLCT transition, and thatthese absorptions overlap each other to form a broad absorption band onthe longest wavelength side in the absorption spectrum. In other words,the difference between the lowest singlet excited state and the lowesttriplet excited state is small, and absorptions based on these statesoverlap each other to form a broad absorption band on the longestwavelength side in the absorption spectrum. Therefore, when anorganometallic complex (especially iridium complex) is used as the guestmaterial, the broad absorption band on the longest wavelength sidelargely overlaps with the phosphorescence spectrum of the host materialas described above, whereby the rate constant k_(h)*_(→g) can beincreased and energy transfer efficiency can be increased.

Furthermore, a fluorescent compound is generally used as the hostmaterial; thus, a phosphorescence lifetime (τ) is a millisecond orlonger which is extremely long (i.e., k_(r)+k_(n) is low). This isbecause the transition from the triplet excited state to the groundstate (singlet) is a forbidden transition. Formula (3) shows that thisis favorable to energy transfer efficiency Φ_(ET).

The above description also suggests that energy transfer from the hostmaterial in the triplet excited state to the guest material in thetriplet excited state, i.e., the process in Formula (2-1), generallytends to occur as long as the phosphorescence spectrum of the hostmaterial overlaps with the absorption spectrum corresponding to thedirect transition of the guest material from the singlet ground state tothe triplet excited state.

(Energy Transfer Efficiency in (2-2))

Next, the energy transfer process in (2-2) is considered. The process inFormula (2-2A) is affected by the efficiency of intersystem crossing ofthe guest material. Therefore, in order to maximize emission efficiency,the process in Formula (2-2B) is considered to be important. SinceDexter mechanism (Formula (2)) is forbidden in this case, only Forstermechanism (Formula (1)) should be considered.

When τ is eliminated from Formula (1) and Formula (3), it can be saidthat the energy transfer efficiency Φ_(ET) is higher when the quantumyield ϕ (here, a fluorescent quantum yield because energy transfer froma singlet excited state is discussed) is higher. However, in practice, amore important factor is that the emission spectrum of the host molecule(here, a fluorescence spectrum because energy transfer from a singletexcited state is discussed) largely overlaps with the absorptionspectrum of the guest molecule (absorption corresponding to the directtransition from the singlet ground state to the triplet excited state)(note that it is preferable that the molar absorption coefficient of theguest molecule be also high). This means that the fluorescence spectrumof the host material overlaps with the absorption band of thephosphorescent compound used as the guest material which is on thelongest wavelength side.

However, this has conventionally been very difficult to achieve. Thereason is that, in order to enable both of the above-described processes(2-1) and (2-2) to occur efficiently, it is clear from the abovediscussion that the host material should be designed so as to have notonly its phosphorescence spectrum but also its fluorescence spectrumoverlapping with the absorption band of the guest material which is onthe longest wavelength side. In other words, the host material should bedesigned so as to have its fluorescence spectrum in a position similarto that of the phosphorescence spectrum.

However, in general, the S₁ level differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used as a host material in a light-emitting element including aphosphorescent compound, has a phosphorescence spectrum at around 500 nmand has a fluorescence spectrum at around 400 nm, which are largelydifferent by about 100 nm. This example also shows that it is extremelydifficult to design a host material so as to have its fluorescencespectrum in a position similar to that of its phosphorescence spectrum.Therefore, it is very important to improve efficiency in energy transferfrom the host material in the singlet excited state to the guestmaterial.

Therefore, one embodiment of the present invention provides a usefultechnique which can overcome such a problem of the efficiency of theenergy transfer from the host material in the singlet excited state tothe guest material. Specific embodiments thereof will be describedbelow.

Embodiment 1

In this embodiment, a structural concept of a light-emitting element inone embodiment of the present invention and a specific structure of thelight-emitting element will be described. Note that a light-emittingelement in one embodiment of the present invention includes, between apair of electrodes (anode and cathode), an EL layer which includes alight-emitting layer. The light-emitting layer has a stacked-layerstructure including a first light-emitting layer and a secondlight-emitting layer. The first light-emitting layer is formed on theanode side and contains at least a first light-emitting substance (guestmaterial) converting triplet excitation energy into light emission, afirst organic compound (host material) having an electron-transportproperty, and a second organic compound (assist material) having ahole-transport property. The second light-emitting layer contains atleast a second light-emitting substance (guest material) convertingtriplet excitation energy into light emission, the first organiccompound (host material) having an electron-transport property, and athird organic compound (assist material) having a hole-transportproperty.

Note that the highest occupied molecular orbital level (HOMO level) ofthe second organic compound (assist material) contained in the firstlight-emitting layer is lower than that of the third organic compound(assist material) contained in the second light-emitting layer.Therefore, excitation energy (E_(A)) of an exciplex formed in the firstlight-emitting layer can be designed to be higher than excitation energy(E_(B)) of an exciplex formed in the second light-emitting layer.

Further, the first light-emitting substance contained in the firstlight-emitting layer is a substance which emits light with a wavelengthshorter than that of light emitted from the second light-emittingsubstance contained in the second light-emitting layer.

First, an element structure of a light-emitting element which is anexample of the present invention is described with reference to FIG. 1A.

In the element structure illustrated in FIG. 1A, an EL layer 103including a light-emitting layer 106 is provided between a pair ofelectrodes (an anode 101 and a cathode 102), and the EL layer 103 has astructure in which a hole-injection layer 104, a hole-transport layer105, the light-emitting layer 106 (106 a and 106 b), anelectron-transport layer 107, an electron-injection layer 108, and thelike are sequentially stacked over the anode 101.

As illustrated in FIG. 1A, the light-emitting layer 106 in oneembodiment of the present invention has a stacked-layer structureincluding the first light-emitting layer 106 a and the secondlight-emitting layer 106 b. The first light-emitting layer 106 a isformed on the anode side and contains at least a first light-emittingsubstance (guest material) 109 a converting triplet excitation energyinto light emission, a first organic compound (host material) 110 havingan electron-transport property, and a second organic compound (assistmaterial) 111 having a hole-transport property. The secondlight-emitting layer 106 b contains at least a second light-emittingsubstance (guest material) 109 b converting triplet excitation energyinto light emission, the first organic compound (host material) 110having an electron-transport property, and a third organic compound(assist material) 112 having a hole-transport property. Anelectron-transport material having an electron mobility of 10⁻⁶ cm²/Vsor more is mainly used as the first organic compound 110, andhole-transport materials having a hole mobility of 10⁻⁶ cm²/Vs or moreare mainly used as the second organic compound 111 and the third organiccompound 112. In this specification, the first organic compound 110 isreferred to as a host material, and the second organic compound 111 andthe third organic compound 112 are referred to as assist materials.

A combination of the first organic compound (host material) 110 and thesecond organic compound (assist material) 111 in the firstlight-emitting layer 106 a forms an exciplex (also referred to as anexcited complex). Further, the emission wavelength of the exciplexformed by the first organic compound (host material) 110 and the secondorganic compound (assist material) 111 is located on the longerwavelength side with respect to the emission wavelength (fluorescentwavelength) of each of the first and second organic compounds (host andassist materials) 110 and 111. Therefore, the fluorescence spectrum ofthe first organic compound (host material) 110 and the fluorescencespectrum of the second organic compound (assist material) 111 can beconverted into an emission spectrum which is located on the longerwavelength side.

The same applies to the second light-emitting layer 106 b. Thus, acombination of the first organic compound (host material) 110 and thethird organic compound (assist material) 112 in the secondlight-emitting layer 106 b forms an exciplex (also referred to as anexcited complex). Further, the emission wavelength of the exciplexformed by the first organic compound (host material) 110 and the thirdorganic compound (assist material) 112 is located on the longerwavelength side with respect to the emission wavelength (fluorescentwavelength) of each of the first and third organic compounds (host andassist materials) 110 and 112. Therefore, the fluorescence spectrum ofthe first organic compound (host material) 110 and the fluorescencespectrum of the third organic compound (assist material) 112 can beconverted into an emission spectrum which is located on the longerwavelength side.

Note that in the above structure, it is preferable that the level of atriplet excitation energy (T₁ level) of each of the first and secondorganic compounds (host and assist materials) 110 and 111 be higher thanthe T₁ level of the first light-emitting substance (guest material) 109a converting triplet excitation energy into light emission. This isbecause, when the T₁ level of the first organic compound 110 (or thesecond organic compound 111) is lower than the T₁ level of the firstlight-emitting substance (guest material) 109 a, the triplet excitationenergy of the first light-emitting substance (guest material) 109 a,which contributes to light emission, is quenched by the first organiccompound 110 (or the second organic compound 111) and accordingly theemission efficiency decreases.

In a similar manner, it is preferable that the level of a tripletexcitation energy (T₁ level) of each of the first and third organiccompounds (host and assist materials) 110 and 112 be higher than the T₁level of the second light-emitting substance (guest material) 109 bconverting triplet excitation energy into light emission. This isbecause, when the T₁ level of the first organic compound 110 (or thethird organic compound 112) is lower than the T₁ level of the secondlight-emitting substance (guest material) 109 b, the triplet excitationenergy of the second light-emitting substance (guest material) 109 b,which contributes to light emission, is quenched by the first organiccompound 110 (or the third organic compound 112) and accordingly theemission efficiency decreases.

Furthermore, in the first light-emitting layer 106 a included in thelight-emitting layer 106, either the first organic compound (hostmaterial) 110 or the second organic compound (assist material) 111 maybe contained in a higher proportion, and in the second light-emittinglayer 106 b included in the light-emitting layer 106, either the firstorganic compound (host material) 110 or the third organic compound(assist material) 112 may be contained in a higher proportion; thepresent invention includes all the cases in its scope.

FIG. 1B is a band diagram which explains energy relation between thefirst organic compound (host material) 110, the second organic compound(assist material) 111, and the third organic compound (assist material)112 in the light-emitting layer 106 (the first light-emitting layer 106a and the second light-emitting layer 106 b) having the above-describedstructure.

As illustrated in FIG. 1B, in the first light-emitting layer 106 a, theexcitation energy (E_(A)) of the exciplex formed by the first organiccompound (host material) 110 and the second organic compound (assistmaterial) 111 is dependent on the HOMO level of the second organiccompound (assist material) 111 and the LUMO level of the first organiccompound (host material) 110. Further, in the second light-emittinglayer 106 b, the excitation energy (E_(B)) of the exciplex formed by thefirst organic compound (host material) 110 and the third organiccompound (assist material) 112 is dependent on the HOMO level of thethird organic compound (assist material) 112 and the LUMO level of thefirst organic compound (host material) 110. Note that the second organiccompound (assist material) 111 contained in the first light-emittinglayer 106 a has a lower HOMO level than the third organic compound(assist material) 112 contained in the second light-emitting layer 106b, which means that the excitation energy (E_(A)) of the exciplex formedin the first light-emitting layer 106 a can be designed to be higherthan the excitation energy (E_(B)) of the exciplex formed in the secondlight-emitting layer 106 b.

Note that owing to the above-described element structure, the exciplexformed in the first light-emitting layer 106 a has higher excitationenergy than the exciplex formed in the second light-emitting layer 106b; therefore, when a substance emitting light with a wavelength shorterthan that of light emitted from the second light-emitting substance(guest material) 109 b which is contained in the second light-emittinglayer 106 b and converts triplet excitation energy into light emissionis used as the first light-emitting substance (guest material) 109 awhich is contained in the first light-emitting layer 106 a and convertstriplet excitation energy into light emission, the first light-emittinglayer 106 a and the second light-emitting layer 106 b can provide lightemission at the same time. Further, part of excitation energy of theexciplex formed in the first light-emitting layer 106 a which does notcontribute to light emission can be utilized as excitation energy forthe second light-emitting substance (guest material) 109 b whichconverts triplet excitation energy into light emission in the secondlight-emitting layer 106 b. Accordingly, emission efficiency in thelight-emitting element can be further increased.

In the light-emitting element described in this embodiment, exciplexesare formed in the light-emitting layer 106 (the first light-emittinglayer 106 a and the second light-emitting layer 106 b), and thefluorescence spectrum of the first organic compound (host material) 110,the fluorescence spectrum of the second organic compound (assistmaterial) 111, or the fluorescence spectrum of the third organiccompound (assist material) 112 can be converted into an emissionspectrum which is located on the longer wavelength side. This means thateven when the fluorescence spectrum of the first organic compound 110,the second organic compound 111, or the third organic compound 112 islocated on the shorter wavelength side with respect to the absorptionband on the longest wavelength side of the light-emitting substance 109(the first light-emitting substance (guest material) 109 a and thesecond light-emitting substance (guest material) 109 b) convertingtriplet excitation energy into light emission as illustrated in FIG. 2,or does not have an overlap with the absorption band on the longestwavelength side of the light-emitting substance 109 (the firstlight-emitting substance (guest material) 109 a and the secondlight-emitting substance (guest material) 109 b) converting tripletexcitation energy into light emission, the emission spectrum of theexciplex and the absorption band can have an increased overlap.Accordingly, the energy transfer efficiency in Formula (2-2B) above canbe increased.

Furthermore, the exciplex is considered to have an extremely smalldifference between singlet excited energy and triplet excited energy. Inother words, the emission spectrum of the exciplex from the singletstate and the emission spectrum thereof from the triplet state arehighly close to each other. Accordingly, in the case where design isimplemented such that the emission spectrum of the exciplex (generallythe emission spectrum of the exciplex from the singlet state) overlapswith the absorption band on the longest wavelength side of thelight-emitting substance 109 converting triplet excitation energy intolight emission as described above, the emission spectrum of the exciplexfrom the triplet state (which is not observed at room temperature andalso not observed in many cases at low temperature) also overlaps withthe absorption band on the longest wavelength side of the light-emittingsubstance 109 converting triplet excitation energy into light emission.In other words, not only the efficiency of the energy transfer from thesinglet excited state ((2-2)) but also the efficiency of the energytransfer from the triplet excited state ((2-1)) can be increased, and asa result, energy from both the singlet and triplet excited states can beefficiently converted into light emission.

Thus, molecular orbital calculations were performed as described belowto verify whether or not an exciplex actually has such characteristics.In general, a combination of a heteroaromatic compound and an aromaticamine often forms an exciplex under the influence of the lowestunoccupied molecular orbital (LUMO) level of the heteroaromatic compoundwhich is deeper than the LUMO level of the aromatic amine (the propertyof easily accepting electrons) and the highest occupied molecularorbital (HOMO) level of the aromatic amine which is shallower than theHOMO level of the heteroaromatic compound (the property of easilyaccepting holes). Thus, calculations were performed using a combinationof dibenzo[fh]quinoxaline (abbreviation: DBq), which is a typicalskeleton forming the LUMO level of a heteroaromatic compound and is amodel of the first organic compound 110 in one embodiment of the presentinvention, and triphenylamine (abbreviation: TPA), which is a typicalskeleton forming the HOMO level of an aromatic amine and is a model ofthe second organic compound 111 (or the third organic compound 112) inone embodiment of the present invention.

First, the optimal molecular structures and the excitation energies ofone molecule of DBq (abbreviation) and one molecule of TPA(abbreviation) in the lowest singlet excited state (S₁) and the lowesttriplet excited state (T₁) were calculated using the time-dependentdensity functional theory (TD-DFT). Furthermore, the excitation energyof a dimer of DBq (abbreviation) and TPA (abbreviation) was alsocalculated.

In the DFT (density functional theory), the total energy is representedas the sum of potential energy, electrostatic energy between electrons,electronic kinetic energy, and exchange-correlation energy including allthe complicated interactions between electrons. Also in the DFT, anexchange-correlation interaction is approximated by a functional (afunction of another function) of one electron potential represented interms of electron density to enable high-speed and high-accuracycalculations. Here, B3LYP which was a hybrid functional was used tospecify the weight of each parameter related to exchange-correlationenergy.

In addition, as a basis function, 6-311 (a basis function of atriple-split valence basis set using three contraction functions foreach valence orbital) was applied to all the atoms.

By the above basis function, for example, is to 3s orbitals areconsidered in the case of hydrogen atoms, while 1s to 4s and 2p to 4porbitals are considered in the case of carbon atoms. Furthermore, toimprove calculation accuracy, the p function and the d function aspolarization basis sets were added to hydrogen atoms and atoms otherthan hydrogen atoms, respectively.

Note that Gaussian 09 was used as a quantum chemistry computationalprogram. A high performance computer (Altix 4700, manufactured by SGIJapan, Ltd.) was used for the calculations.

First, the HOMO levels and the LUMO levels of one molecule of DBq(abbreviation), one molecule of TPA (abbreviation), and a dimer of DBq(abbreviation) and TPA (abbreviation) were calculated. FIG. 3 shows theHOMO levels and the LUMO levels, and FIGS. 4A1, 4A2, 4B1, 4B2, 4C1, and4C2 show HOMO and LUMO distributions.

FIG. 4A1 shows the LUMO distribution of one molecule of DBq(abbreviation); FIG. 4A2, the HOMO distribution of one molecule of DBq(abbreviation); FIG. 4B1, the LUMO distribution of one molecule of TPA(abbreviation); FIG. 4B2, the HOMO distribution of one molecule of TPA(abbreviation); FIG. 4C1, the LUMO distribution of the dimer of DBq(abbreviation) and TPA (abbreviation); and FIG. 4C2, the HOMOdistribution of the dimer of DBq (abbreviation) and TPA (abbreviation).

As shown in FIG. 3, it is suggested that the dimer of DBq (abbreviation)and TPA (abbreviation) forms an exciplex of DBq (abbreviation) and TPA(abbreviation) under the influence of the LUMO level (−1.99 eV) of DBq(abbreviation) which is deeper (lower) than the LUMO level of TPA(abbreviation) and the HOMO level (−5.21 eV) of TPA (abbreviation) whichis shallower (higher) than the HOMO level of DBq (abbreviation). Infact, as is clear from FIGS. 4C1 and 4C2, the LUMO of the dimer of DBq(abbreviation) and TPA (abbreviation) is distributed on the DBq(abbreviation) side, and the HOMO thereof is distributed on the TPA(abbreviation) side.

Next, excitation energies obtained from the optimal molecular structuresof one molecule of DBq (abbreviation) at S₁ and T₁ levels will be shown.Here, the excitation energies at the S₁ and T₁ levels correspond tofluorescence and phosphorescence wavelengths, respectively, obtainedfrom one molecule of DBq (abbreviation). The excitation energy at the S₁level of one molecule of DBq (abbreviation) is 3.294 eV, and thefluorescence wavelength is 376.4 nm. The excitation energy at the T₁level of one molecule of DBq (abbreviation) is 2.460 eV, and thephosphorescence wavelength is 504.1 nm.

In addition, excitation energies obtained from the optimal molecularstructures of one molecule of TPA (abbreviation) at S₁ and T₁ levelswill be shown. Here, the excitation energies at the S₁ and T₁ levelscorrespond to fluorescence and phosphorescence wavelengths,respectively, obtained from one molecule of TPA (abbreviation). Theexcitation energy at the S₁ level of one molecule of TPA (abbreviation)is 3.508 eV, and the fluorescence wavelength is 353.4 nm. The excitationenergy at the T₁ level of one molecule of TPA (abbreviation) is 2.610eV, and the phosphorescence wavelength is 474.7 nm.

Furthermore, excitation energies obtained from the optimal molecularstructures of the dimer of DBq (abbreviation) and TPA (abbreviation) atS₁ and T₁ levels will be shown. The excitation energies at the S₁ and T₁levels correspond to fluorescence and phosphorescence wavelengths,respectively, obtained from the dimer of DBq (abbreviation) and TPA(abbreviation). The excitation energy at the S₁ level of the dimer ofDBq (abbreviation) and TPA (abbreviation) is 2.036 eV, and thefluorescence wavelength is 609.1 nm. The excitation energy at the T₁level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.030eV, and the phosphorescence wavelength is 610.0 nm.

It is found from the above that each of the phosphorescence wavelengthsof one molecule of DBq (abbreviation) and one molecule of TPA(abbreviation) is shifted to the longer wavelength side by about 100 nm.This result shows a tendency similar to that of CBP (abbreviation)(measured values) described above and supports the validity of thecalculations.

On the other hand, it is found that the fluorescence wavelength of thedimer of DBq (abbreviation) and TPA (abbreviation) is located on thelonger wavelength side with respect to the fluorescence wavelengths ofone molecule of DBq (abbreviation) and one molecule of TPA(abbreviation). It is also found that the difference between thefluorescence wavelength and the phosphorescence wavelength of the dimerof DBq (abbreviation) and TPA (abbreviation) is only 0.9 nm and thatthese wavelengths are substantially the same.

These results indicate that the singlet excitation energy and thetriplet excitation energy of the exciplex are substantially equivalent.Therefore, it is indicated as described above that the exciplex canefficiently transfer energy from both the singlet state and the tripletstate to the light-emitting substance (the guest materials including thefirst light-emitting substance and the second light-emitting substance)which converts triplet excitation energy into light emission.

In the above manner, the light-emitting element in one embodiment of thepresent invention transfers energy by utilizing an overlap between theemission spectrum of the exciplex formed in the light-emitting layer andthe absorption spectrum of the light-emitting substance (the guestmaterials including the first light-emitting substance and the secondlight-emitting substance) which converts triplet excitation energy intolight emission and thus has high energy transfer efficiency. Therefore,the light-emitting element can achieve high external quantum efficiency.

In addition, the exciplex exists only in an excited state and thus hasno ground state capable of absorbing energy. Therefore, a phenomenon inwhich the light-emitting substance (guest material) converting tripletexcitation energy into light emission is deactivated by energy transferfrom the light-emitting substance (guest material) converting tripletexcitation energy into light emission in the singlet excited state andtriplet excited state to the exciplex before light emission (i.e.,emission efficiency is lowered) is not considered to occur in principle.This also contributes to improvement of external quantum efficiency.

Note that the above-described exciplex is formed by an interactionbetween dissimilar molecules in excited states. The exciplex isgenerally known to be easily formed between a material having arelatively deep LUMO level and a material having a relatively shallowHOMO level.

An emission wavelength of the exciplex depends on a difference in energybetween the HOMO level and the LUMO level. As a general tendency, whenthe energy difference is large, the emission wavelength is short, andwhen the energy difference is small, the emission wavelength is long.

Therefore, the HOMO levels and LUMO levels of the first organic compound(host material) 110, the second organic compound (assist material) 111,and the third organic compound (assist material) 112 in this embodimentare different from each other as illustrated in FIG. 1B. Specifically,the energy levels vary in the following order: the HOMO level of thefirst organic compound 110<the HOMO levels of the second organiccompound 111 and the third organic compound 112<the LUMO level of thefirst organic compound 110<the LUMO levels of the second organiccompound 111 and the third organic compound 112.

In each of the light-emitting layers, when the exciplex is formed by thetwo organic compounds (the first organic compound 110 and the secondorganic compound 111 in the first light-emitting layer 106 a, and thefirst organic compound 110 and the third organic compound 112 in thesecond light-emitting layer 106 b), the LUMO levels of the exciplexes inthe first light-emitting layer 106 a and the second light-emitting layer106 b depend on the first organic compound (host material) 110, the HOMOlevel of the exciplex in the first light-emitting layer 106 a depends onthe second organic compound (assist material) 111, and the HOMO level ofthe exciplex in the second light-emitting layer 106 b depends on theHOMO level of the third organic compound (assist material) 112.Therefore, the excitation energy (E_(A)) of the exciplex in the firstlight-emitting layer 106 a is higher than the excitation energy (E_(B))of the exciplex in the second light-emitting layer 106 b. In otherwords, when a substance emitting light with a wavelength shorter thanthat of light emitted from the second light-emitting substance 109 bwhich is contained in the second light-emitting layer and convertstriplet excitation energy into light emission is used as the firstlight-emitting substance 109 a which is contained in the firstlight-emitting layer and converts triplet excitation energy into lightemission, a light-emitting element with high emission efficiency can beformed. Moreover, light-emitting materials with different emissionwavelengths can efficiently emit light at the same time.

Note that the process of the exciplex formation in one embodiment of thepresent invention can be either of the following two processes.

One formation process is that an exciplex is formed from the second andthird organic compounds (assist materials) having carriers(specifically, cation).

In general, when an electron and a hole recombine in a host material,excitation energy is transferred from the host material in an excitedstate to a guest material, whereby the guest material is brought into anexcited state to emit light. Before the excitation energy is transferredfrom the host material to the guest material, the host material itselfemits light or the excitation energy turns into thermal energy, whichleads to partial deactivation of the excitation energy. In particular,when the host material is in a singlet excited state, energy transferdoes not readily occur as described in (2-2). Such deactivation ofexcitation energy is one of causes for a decrease in lifetime of alight-emitting element.

However, in one embodiment of the present invention, an exciplex isformed from the first organic compound (host material) and the secondorganic compound (or the third organic compound) (assist material)having carriers (cation or anion); therefore, formation of a singletexciton of the first organic compound (host material) can be suppressed.In other words, there can be a process where an exciplex is directlyformed without formation of a singlet exciton. Thus, deactivation of thesinglet excitation energy can be inhibited. Accordingly, alight-emitting element having a long lifetime can be obtained.

For example, in the case where the first organic compound is anelectron-trapping compound having the property of easily capturingelectrons (carrier) (having a deep LUMO level) among electron-transportmaterials and the second organic compound (or the third organiccompound) is a hole-trapping compound having the property of easilycapturing holes (carrier) (having a shallow HOMO level) amonghole-transport materials, an exciplex is formed directly from an anionof the first organic compound and a cation of the second organiccompound (or the third organic compound). An exciplex formed throughsuch a process is particularly referred to as an electroplex. Alight-emitting element having high emission efficiency can be obtainedby suppressing the generation of the singlet excited state of the firstorganic compound (host material) and transferring energy from anelectroplex to the light-emitting substance (guest material) whichconverts triplet excitation energy into light emission, in theabove-described manner. Note that in this case, the generation of thetriplet excited state of the first organic compound (host material) issimilarly suppressed and an exciplex is directly formed; therefore,energy transfer is considered to occur from the exciplex to thelight-emitting substance (guest material) which converts tripletexcitation energy into light emission.

The other formation process is an elementary process where one of thehost material (the first organic compound) and the assist material (thesecond organic compound or the third organic compound) forms a singletexciton and then interacts with the other in the ground state to form anexciplex. Unlike an electroplex, a singlet excited state of the firstorganic compound (host material), the second organic compound (assistmaterial), or the third organic compound (assist material) istemporarily generated in this case, but this is rapidly converted intoan exciplex, and thus, deactivation of singlet excitation energy can beinhibited. Thus, it is possible to inhibit deactivation of excitationenergy of the first organic compound (host material), the second organiccompound (assist material), or the third organic compound (assistmaterial). Note that in this case, it is considered that the tripletexcited state of the host material is similarly rapidly converted intoan exciplex and energy is transferred from the exciplex to thelight-emitting substance (guest material) which converts tripletexcitation energy into light emission.

Note that in the case where the first organic compound (host material)is an electron-trapping compound, the second organic compound (or thethird organic compound) (assist material) is a hole-trapping compound,and the difference between the HOMO levels and the difference betweenthe LUMO levels of these compounds are large (specifically, 0.3 eV ormore), electrons are selectively injected into the first organiccompound (host material) and holes are selectively injected into thesecond organic compound (or the third organic compound) (assistmaterial). In this case, it is thought that the process where anelectroplex is formed takes precedence over the process where anexciplex is formed through a singlet exciton.

To make the emission spectrum of the exciplex and the absorptionspectrum of the light-emitting substance (guest material) convertingtriplet excitation energy into light emission sufficiently overlap eachother, the difference between the energy of a peak of the emissionspectrum and the energy of a peak of the absorption band on the lowestenergy side in the absorption spectrum is preferably 0.3 eV or less. Thedifference is more preferably 0.2 eV or less, even more preferably 0.1eV or less.

In the light-emitting element in one embodiment of the presentinvention, it is also preferable that the excitation energy of theexciplex be sufficiently transferred to the light-emitting substance(guest material) which converts triplet excitation energy into lightemission, and that light emission from the exciplex be not substantiallyobserved. Therefore, it is preferable that energy be transferred to thelight-emitting substance which converts triplet excitation energy intolight emission through an exciplex so that the light-emitting substancewhich converts triplet excitation energy into light emission emitslight. Note that as the light-emitting substance which converts tripletexcitation energy into light emission, a phosphorescent compound (e.g.,an organometallic complex), a thermally activated delayed fluorescence(TADF) material, or the like is preferably used.

In the case where a light-emitting substance which converts tripletexcitation energy into light emission is used as the first organiccompound (host material) in the first light emitting layer 106 a of thelight-emitting element in one embodiment of the present invention, thefirst organic compound (host material) itself readily emits light anddoes not readily allow energy to be transferred to the light-emittingsubstance (guest material) which converts triplet excitation energy intolight emission. In this case, it is favorable if the first organiccompound could emit light efficiently, but it is difficult to achievehigh emission efficiency because a high concentration of the hostmaterial may cause a problem of concentration quenching. Therefore, thecase where at least one of the first organic compound (host material)and the second organic compound (or the third organic compound) (assistmaterial) is a fluorescent compound (i.e., a compound which readilyundergoes light emission or thermal deactivation from the singletexcited state) is effective. For this reason, it is preferable that atleast one of the first organic compound (host material) and the secondorganic compound (or the third organic compound) (assist material) be afluorescent compound and an exciplex be used as a medium for energytransfer.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, an example of a light-emitting element in oneembodiment of the present invention is described with reference to FIG.5.

In the light-emitting element described in this embodiment, asillustrated in FIG. 5, an EL layer 203 including a light-emitting layer206 is provided between a pair of electrodes (a first electrode (anode)201 and a second electrode (cathode) 202), and the EL layer 203 includesa hole-injection layer 204, a hole-transport layer 205, anelectron-transport layer 207, an electron-injection layer 208, and thelike in addition to the light-emitting layer 206 having a stacked-layerstructure including a first light-emitting layer 206 a and a secondlight-emitting layer 206 b.

Note that the light-emitting layer 206 in this embodiment has astacked-layer structure including the first light-emitting layer 206 aand the second light-emitting layer 206 b. In the light-emitting layer206, the first light-emitting layer 206 a contains a firstlight-emitting substance (guest material) 209 a converting tripletexcitation energy into light emission, a first organic compound (hostmaterial) 210 having an electron-transport property, and a secondorganic compound (assist material) 211 having a hole-transport property;and the second light-emitting layer 206 b contains a secondlight-emitting substance (guest material) 209 b converting tripletexcitation energy into light emission, the first organic compound (hostmaterial) 210 having an electron-transport property, and the thirdorganic compound (assist material) 212 having a hole-transport property.

Note that the HOMO level of the second organic compound (assistmaterial) contained in the first light-emitting layer is lower than thatof the third organic compound (assist material) contained in the secondlight-emitting layer. Therefore, excitation energy (E_(A)) of anexciplex formed in the first light-emitting layer can be designed to behigher than excitation energy (E_(B)) of an exciplex formed in thesecond light-emitting layer.

Note that crystallization of the light-emitting layer 206 (the firstlight-emitting layer 206 a and the second light-emitting layer 206 b)can be suppressed by employing a structure in which the firstlight-emitting substance 209 a converting triplet excitation energy intolight emission is dispersed in the first organic compound (hostmaterial) 210 and the second organic compound (assist material) 211 inthe first light-emitting layer 206 a, and the second light-emittingsubstance 209 b converting triplet excitation energy into light emissionis dispersed in the first organic compound (host material) 210 and thethird organic compound (assist material) 212 in the secondlight-emitting layer 206 b. Further, it is possible to suppressconcentration quenching due to high concentration of the light-emittingsubstance 209 (209 a and 209 b), and thus the light-emitting element canhave higher emission efficiency.

It is preferable that the level of a triplet excitation energy (T₁level) of each of the first organic compound 210, the second organiccompound 211, and the third organic compound 212 be higher than the T₁level of the light-emitting substance 209 (209 a and 209 b) convertingtriplet excitation energy into light emission. This is because, when theT₁ level of each of the first organic compound 210, the second organiccompound 211, and the third organic compound 212 is lower than the T₁level of the light-emitting substance 209 (209 a and 209 b) convertingtriplet excitation energy into light emission, the triplet excitationenergy of the light-emitting substance 209 (209 a and 209 b) convertingtriplet excitation energy into light emission, which contributes tolight emission, is quenched by the first organic compound 210 (or thesecond organic compound 211, or the third organic compound 212) andaccordingly the emission efficiency decreases.

In the light-emitting layer 206 in this embodiment, at the time ofrecombination of carriers (electrons and holes) injected from therespective electrodes, the first organic compound 210 and the secondorganic compound 211 form an exciplex in the first light-emitting layer206 a, and the first organic compound 210 and the third organic compound212 form an exciplex in the second light-emitting layer 206 b. Thus, afluorescence spectrum of the first organic compound 210 and that of thesecond organic compound 211 in the first light-emitting layer 206 a canbe converted into an emission spectrum of the exciplex which is locatedon a longer wavelength side, and a fluorescence spectrum of the firstorganic compound 210 and that of the third organic compound 212 in thesecond light-emitting layer 206 b can be converted into an emissionspectrum of the exciplex which is located on a longer wavelength side.In consideration of this, the first organic compound 210 and the secondorganic compound 211 in the first light-emitting layer 206 a and thefirst organic compound 210 and the third organic compound 212 in thesecond light-emitting layer 206 b are selected to make an overlapbetween the emission spectrum of the exciplex and the absorptionspectrum of the light-emitting substance (guest material) 209 convertingtriplet excitation energy into light emission large and thereby tomaximize energy transfer from a singlet excited state. That is, it isassumed here that energy transfer from the exciplex, not the hostmaterial, occurs also in the case of a triplet excited state.

Note that as the light-emitting substance 209 (the first light-emittingsubstance 209 a and the second light-emitting substance 209 b) whichconverts triplet excitation energy into light emission, a phosphorescentcompound (e.g., an organometallic complex), a thermally activateddelayed fluorescence (TADF) material, or the like is preferably used. Anelectron-transport material is preferably used as the first organiccompound (host material) 210. Hole-transport materials are preferablyused as the second organic compound (assist material) 211 and the thirdorganic compound (assist material) 212.

Note that examples of the organometallic complex includebis[2-(4′,6′-difluorophenyl)pyridinato-N, C²′]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N, C^(2′)]iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation: Flracac),tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate(abbreviation: Ir(bzq)₂(acac)),bis(2,4-diphenyl-1,3-oxazolato-N,C²′)iridium(III) acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C²′)iridium(III) acetylacetonate(abbreviation: Ir(bt)₂(acac)),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C²′)iridium(III) acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)).

As the electron-transport material, a π-electron deficientheteroaromatic compound such as a nitrogen-containing heteroaromaticcompound is preferable, examples of which include quinoxalinederivatives and dibenzoquinoxaline derivatives such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), and6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:6mDBTPDBq-II).

As the hole-transport material, a π-electron rich heteroaromaticcompound (e.g., a carbazole derivative or an indole derivative) or anaromatic amine compound is preferable, examples of which include4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), and3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2).

Note that materials which can be used for the light-emitting substance(guest material) 209 (the first light-emitting substance 209 a and thesecond light-emitting substance 209 b) converting triplet excitationenergy into light emission, the first organic compound (host material)210, the second organic compound (assist material) 211, and the thirdorganic compound (assist material) 212 are not limited to the aboveexamples. The combination is determined so that an exciplex can beformed, the emission spectrum of the exciplex overlaps with theabsorption spectrum of the light-emitting substance (guest material) 209(the first light-emitting substance 209 a or the second light-emittingsubstance 209 b) converting triplet excitation energy into lightemission, and the peak of the emission spectrum of the exciplex has alonger wavelength than the peak of the absorption spectrum of thelight-emitting substance (guest material) 209 (the first light-emittingsubstance 209 a or the second light-emitting substance 209 b) convertingtriplet excitation energy into light emission.

In the case where an electron-transport material is used as the firstorganic compound 210 and a hole-transport material is used as the secondorganic compound 211, carrier balance can be controlled by the mixtureratio of the compounds. Specifically, the ratio of the first organiccompound 210 to the second organic compound 211 is preferably 1:9 to9:1.

A specific example in which the light-emitting element described in thisembodiment is manufactured is described below.

For the first electrode (anode) 201 and the second electrode (cathode)202, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like can be used. Specifically, indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used.In addition, an element belonging to Group 1 or Group 2 of the periodictable, for example, an alkali metal such as lithium (Li) or cesium (Cs),an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr), an alloy containing such an element (e.g., MgAg orAlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), analloy containing such an element, graphene, or the like can be used. Thefirst electrode (anode) 201 and the second electrode (cathode) 202 canbe formed by, for example, a sputtering method, an evaporation method(including a vacuum evaporation method), or the like.

Examples of a substance having a high hole-transport property which isused for the hole-injection layer 204 and the hole-transport layer 205include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Other examples include carbazole derivativessuch as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA).The substances mentioned here are mainly substances that have a holemobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances,any substance that has a property of transporting more holes thanelectrons may be used.

Still other examples include high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD).

Further, examples of an acceptor substance which can be used for thehole-injection layer 204 include oxides of transition metals, oxides ofmetals belonging to Groups 4 to 8 of the periodic table, and the like.Specifically, molybdenum oxide is particularly preferable.

The light-emitting layer 206 (206 a and 206 b) is formed such that, asdescribed above, the first light-emitting layer 206 a contains at leastthe first light-emitting substance 209 a, the first organic compound(host material) 210, and the second organic compound (assist material)211, and the second light-emitting layer 206 b contains at least thesecond light-emitting substance 209 b, the first organic compound (hostmaterial) 210, and the third organic compound (assist material) 212.

The electron-transport layer 207 is a layer that contains a substancehaving a high electron-transport property. For the electron-transportlayer 207, it is possible to use a metal complex such as Alq₃,tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq,Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂). Alternatively, it is possible to use a heteroaromaticcompound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,24-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Furtheralternatively, it is possible to use a high molecular compound such aspoly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy). The substances mentioned here are mainlysubstances that have an electron mobility of 10⁻⁶ cm²/Vs or more. Notethat other than these substances, any substance that has a property oftransporting more electrons than holes may be used for theelectron-transport layer 207.

The electron-transport layer 207 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 208 is a layer that contains a substancehaving a high electron-injection property. Examples of the substancethat can be used for the electron-injection layer 208 include alkalimetals, alkaline earth metals, and compounds thereof, such as lithiumfluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂), andlithium oxide (LiO_(x)), and rare earth metal compounds, such as erbiumfluoride (ErF₃). Alternatively, the above-mentioned substances forforming the electron-transport layer 207 can be used.

Alternatively, a composite material in which an organic compound and anelectron donor (a donor) are mixed may be used for theelectron-injection layer 208. Such a composite material, in whichelectrons are generated in the organic compound by the electron donor,has high electron-injection and electron-transport properties. Theorganic compound here is preferably a material excellent in transportingthe generated electrons, and specifically any of the above substances(such as metal complexes and heteroaromatic compounds) for theelectron-transport layer 207 can be used. As the electron donor, asubstance showing an electron-donating property with respect to theorganic compound may be used. Specifically, alkali metals, alkalineearth metals, and rare earth metals are preferable, and lithium, cesium,magnesium, calcium, erbium, ytterbium, and the like can be given. Any ofalkali metal oxides and alkaline earth metal oxides is preferable,examples of which are lithium oxide, calcium oxide, barium oxide, andthe like, and a Lewis base such as magnesium oxide or an organiccompound such as tetrathiafulvalene (abbreviation: TTF) can be used.

Note that the hole-injection layer 204, the hole-transport layer 205,the light-emitting layer 206 (206 a and 206 b), the electron-transportlayer 207, and the electron-injection layer 208 which are mentionedabove can each be formed by a method such as an evaporation method(including a vacuum evaporation method), an inkjet method, or a coatingmethod.

Light emission obtained in the light-emitting layer 206 of theabove-described light-emitting element is extracted to the outsidethrough either the first electrode 201 or the second electrode 202 orboth. Therefore, either the first electrode 201 or the second electrode202 in this embodiment, or both, is an electrode having alight-transmitting property.

In the light-emitting element described in this embodiment, energytransfer efficiency can be improved owing to energy transfer utilizingan overlap between an emission spectrum of an exciplex and an absorptionspectrum of a light-emitting substance (guest material) which convertstriplet excitation energy into light emission; accordingly, thelight-emitting element can achieve high external quantum efficiency.

Note that the light-emitting element described in this embodiment is oneembodiment of the present invention and is particularly characterized bythe structure of the light-emitting layer. Therefore, when the structuredescribed in this embodiment is employed, a passive matrixlight-emitting device, an active matrix light-emitting device, and thelike can be manufactured. Each of these light-emitting devices isincluded in the present invention.

Note that there is no particular limitation on the structure of a TFT inthe case of manufacturing the active matrix light-emitting device. Forexample, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe formed using both an n-type TFT and a p-type TFT or either an n-typeTFT or a p-type TFT. Furthermore, there is also no particular limitationon the crystallinity of a semiconductor film used for the TFT. Forexample, an amorphous semiconductor film, a crystalline semiconductorfilm, an oxide semiconductor film, or the like can be used.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a charge generation layer is provided between aplurality of EL layers will be described.

The light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 302(1) and a second EL layer 302(2)) between a pair of electrodes(a first electrode 301 and a second electrode 304) as illustrated inFIG. 6A.

In this embodiment, the first electrode 301 functions as an anode, andthe second electrode 304 functions as a cathode. Note that the firstelectrode 301 and the second electrode 304 can have structures similarto those described in Embodiment 1. In addition, all or any of theplurality of EL layers (the first EL layer 302(1) and the second ELlayer 302(2)) may have structures similar to those described inEmbodiment 1 or 2. In other words, the structures of the first EL layer302(1) and the second EL layer 302(2) may be the same or different fromeach other and can be similar to those of the EL layers described inEmbodiment 1 or 2.

Further, a charge generation layer (I) 305 is provided between theplurality of EL layers (the first EL layer 302(1) and the second ELlayer 302(2)). The charge generation layer (I) 305 has a function ofinjecting electrons into one of the EL layers and injecting holes intothe other of the EL layers when a voltage is applied between the firstelectrode 301 and the second electrode 304. In this embodiment, when avoltage is applied such that the potential of the first electrode 301 ishigher than that of the second electrode 304, the charge generationlayer (I) 305 injects electrons into the first EL layer 302(1) andinjects holes into the second EL layer 302(2).

Note that in terms of light extraction efficiency, the charge generationlayer (I) 305 preferably has a light-transmitting property with respectto visible light (specifically, the charge generation layer (I) 305preferably has a visible light transmittance of 40% or more). Further,the charge generation layer (I) 305 functions even if it has lowerconductivity than the first electrode 301 or the second electrode 304.

The charge generation layer (I) 305 may have either a structure in whichan electron acceptor (acceptor) is added to an organic compound having ahigh hole-transport property or a structure in which an electron donor(donor) is added to an organic compound having a high electron-transportproperty. Alternatively, both of these structures may be stacked.

In the case where the electron acceptor is added to the organic compoundhaving a high hole-transport property, examples of the organic compoundhaving a high hole-transport property include aromatic amine compoundssuch as NPB, TPD, TDATA, MTDATA, and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and the like. The substances mentioned here aremainly substances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Notethat other than these substances, any organic compound that has aproperty of transporting more holes than electrons may be used.

Examples of the electron acceptor include7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, oxides of transition metals, and oxides of metalsthat belong to Groups 4 to 8 of the periodic table. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable because of their high electron-accepting property. Amongthese, molybdenum oxide is especially preferable since it is stable inthe air, has a low hygroscopic property, and is easy to handle.

On the other hand, in the case where the electron donor is added to theorganic compound having a high electron-transport property, examples ofthe organic compound having a high electron-transport property which canbe used are metal complexes having a quinoline skeleton or abenzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, and BAlq, and thelike. Other examples are metal complexes having an oxazole-based orthiazole-based ligand, such as Zn(BOX)₂ and Zn(BTZ)₂. Other than metalcomplexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thesubstances mentioned here are mainly substances that have an electronmobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances,any organic compound that has a property of transporting more electronsthan holes may be used.

Examples of the electron donor which can be used are alkali metals,alkaline earth metals, rare earth metals, metals that belong to Group 13of the periodic table, and oxides or carbonates thereof. Specifically,lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb),indium (In), lithium oxide, cesium carbonate, and the like arepreferable. An organic compound, such as tetrathianaphthacene, may beused as the electron donor.

Note that forming the charge generation layer (I) 305 by using any ofthe above materials can suppress a drive voltage increase caused by thestack of the EL layers.

Although this embodiment shows the light-emitting element having two ELlayers, the present invention can be similarly applied to alight-emitting element in which n EL layers (n is 3 or more) are stackedas illustrated in FIG. 6B. In the case where a plurality of EL layers isincluded between a pair of electrodes as in the light-emitting elementaccording to this embodiment, by provision of the charge generationlayers (I) between the EL layers, light emission in a high luminanceregion can be obtained with current density kept low. Since the currentdensity can be kept low, the element can have a long lifetime. When thelight-emitting element is applied to lighting, voltage drop due toresistance of an electrode material can be reduced, thereby achievinghomogeneous light emission in a large area. Moreover, it is possible toachieve a light-emitting device which can be driven at a low voltage andhas low power consumption.

Furthermore, by making emission colors of EL layers different, light ofa desired color can be obtained from the light-emitting element as awhole. For example, the emission colors of first and second EL layersare complementary in a light-emitting element having the two EL layers,so that the light-emitting element can be made to emit white light as awhole. Note that the term “complementary” means color relationship inwhich an achromatic color is obtained when colors are mixed. That is,emission of white light can be obtained by mixture of light emitted fromsubstances whose emission colors are complementary colors.

Further, the same applies to a light-emitting element having three ELlayers. For example, the light-emitting element as a whole can emitwhite light when the emission color of the first EL layer is red, theemission color of the second EL layer is green, and the emission colorof the third EL layer is blue.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a light-emitting device which is one embodiment ofthe present invention will be described.

A light-emitting device described in this embodiment has a micro opticalresonator (microcavity) structure in which a light resonant effectbetween a pair of electrodes is utilized. The light-emitting deviceincludes a plurality of light-emitting elements each of which has atleast an EL layer 405 between a pair of electrodes (a reflectiveelectrode 401 and a semi-transmissive and semi-reflective electrode 402)as illustrated in FIG. 7. Further, the EL layer 405 includes at leastlight-emitting layers 404 (404R, 404G, and 404B) each serving as alight-emitting region and may further include a hole-injection layer, ahole-transport layer, an electron-transport layer, an electron-injectionlayer, a charge generation layer (E), and the like. Note that thelight-emitting layers 404 (404R, 404G, and 404B) may have the structureof the light-emitting layer according to one embodiment of the presentinvention which is described in Embodiment 1 or 2.

In this embodiment, a light-emitting device is described which includeslight-emitting elements (a first light-emitting element (R) 410R, asecond light-emitting element (G) 410G, and a third light-emittingelement (B) 410B) having different structures as illustrated in FIG. 7.

The first light-emitting element (R) 410R has a structure in which afirst transparent conductive layer 403 a; an EL layer 405 including afirst light-emitting layer (B) 404B, a second light-emitting layer (G)404G, and a third light-emitting layer (R) 404R in part; and asemi-transmissive and semi-reflective electrode 402 are sequentiallystacked over a reflective electrode 401. The second light-emittingelement (G) 410G has a structure in which a second transparentconductive layer 403 b, the EL layer 405, and the semi-transmissive andsemi-reflective electrode 402 are sequentially stacked over thereflective electrode 401. The third light-emitting element (B) 410B hasa structure in which the EL layer 405 and the semi-transmissive andsemi-reflective electrode 402 are sequentially stacked over thereflective electrode 401.

Note that the reflective electrode 401, the EL layer 405, and thesemi-transmissive and semi-reflective electrode 402 are common to thelight-emitting elements (the first light-emitting element (R) 410R, thesecond light-emitting element (G) 410G, and the third light-emittingelement (B) 410B). The first light-emitting layer (B) 404B emits light(λ_(B)) having a peak in a wavelength region from 420 nm to 480 nm. Thesecond light-emitting layer (G) 404G emits light (λ_(G)) having a peakin a wavelength region from 500 nm to 550 nm. The third light-emittinglayer (R) 404R emits light (λ_(R)) having a peak in a wavelength regionfrom 600 nm to 760 nm. Thus, in each of the light-emitting elements (thefirst light-emitting element (R) 410R, the second light-emitting element(G) 410G, and the third light-emitting element (B) 410B), light emittedfrom the first light-emitting layer (B) 404B, light emitted from thesecond light-emitting layer (G) 404G, and light emitted from the thirdlight-emitting layer (R) 404R overlap with each other; accordingly,light having a broad emission spectrum that covers a visible lightregion can be emitted. Note that the above wavelengths satisfy therelation of λ_(B)<λ_(G)<λ_(R).

Each of the light-emitting elements described in this embodiment has astructure in which the EL layer 405 is provided between the reflectiveelectrode 401 and the semi-transmissive and semi-reflective electrode402. Light emitted in all directions from the light-emitting layersincluded in the EL layer 405 is resonated by the reflective electrode401 and the semi-transmissive and semi-reflective electrode 402 whichfunction as a micro optical resonator (microcavity). Note that thereflective electrode 401 is formed using a conductive material havingreflectivity, and a film whose visible light reflectivity is 40% to100%, preferably 70% to 100%, and whose resistivity is 1×10⁻² Ωcm orlower is used. In addition, the semi-transmissive and semi-reflectiveelectrode 402 is formed using a conductive material having reflectivityand a conductive material having a light-transmitting property, and afilm whose visible light reflectivity is 20% to 80%, preferably 40% to70%, and whose resistivity is 1×10⁻² Ωcm or lower is used.

In this embodiment, the thicknesses of the transparent conductive layers(the first transparent conductive layer 403 a and the second transparentconductive layer 403 b) provided in the first light-emitting element (R)410R and the second light-emitting element (G) 4100 respectively, arevaried between the light-emitting elements, whereby the light-emittingelements differ from each other in the optical path length from thereflective electrode 401 to the semi-transmissive and semi-reflectiveelectrode 402. In other words, in light having a broad emissionspectrum, which is emitted from the light-emitting layers of each of thelight-emitting elements, light with a wavelength that is resonatedbetween the reflective electrode 401 and the semi-transmissive andsemi-reflective electrode 402 can be intensified while light with awavelength that is not resonated therebetween can be attenuated. Thus,when the elements differ from each other in the optical path length fromthe reflective electrode 401 to the semi-transmissive andsemi-reflective electrode 402, light with different wavelengths can beextracted.

Note that the optical path length (also referred to as optical distance)is expressed as a product of an actual distance and a refractive index,and in this embodiment, is a product of an actual thickness and n(refractive index). That is, an optical path length=actual thickness×n.

Further, the total thickness from the reflective electrode 401 to thesemi-transmissive and semi-reflective electrode 402 is set to mλ_(R)/2(m is a natural number) in the first light-emitting element (R) 410R;the total thickness from the reflective electrode 401 to thesemi-transmissive and semi-reflective electrode 402 is set to mλ_(G)/2(m is a natural number) in the second light-emitting element (G) 410G;and the total thickness from the reflective electrode 401 to thesemi-transmissive and semi-reflective electrode 402 is set to mλ_(B)/2(m is a natural number) in the third light-emitting element (B) 410B.

In this manner, the light (λ_(R)) emitted from the third light-emittinglayer (R) 404R included in the EL layer 405 is mainly extracted from thefirst light-emitting element (R) 410R, the light (λ_(G)) emitted fromthe second light-emitting layer (G) 404G included in the EL layer 405 ismainly extracted from the second light-emitting element (G) 410G, andthe light (λ_(B)) emitted from the first light-emitting layer (B) 404Bincluded in the EL layer 405 is mainly extracted from the thirdlight-emitting element (B) 410B. Note that the light extracted from eachof the light-emitting elements is emitted from the semi-transmissive andsemi-reflective electrode 402 side.

Further, strictly speaking, the total thickness from the reflectiveelectrode 401 to the semi-transmissive and semi-reflective electrode 402can be the total thickness from a reflection region in the reflectiveelectrode 401 to a reflection region in the semi-transmissive andsemi-reflective electrode 402. However, it is difficult to preciselydetermine the positions of the reflection regions in the reflectiveelectrode 401 and the semi-transmissive and semi-reflective electrode402; therefore, it is presumed that the above effect can be sufficientlyobtained wherever the reflection regions may be set in the reflectiveelectrode 401 and the semi-transmissive and semi-reflective electrode402.

Next, in the first light-emitting element (R) 410R, the optical pathlength from the reflective electrode 401 to the third light-emittinglayer (R) 404R is adjusted to a desired thickness ((2m′+1)λ_(R)/4, wherem′ is a natural number); thus, light emitted from the thirdlight-emitting layer (R) 404R can be amplified. Light (first reflectedlight) that is reflected by the reflective electrode 401 of the lightemitted from the third light-emitting layer (R) 404R interferes withlight (first incident light) that directly enters the semi-transmissiveand semi-reflective electrode 402 from the third light-emitting layer(R) 404R. Therefore, by adjusting the optical path length from thereflective electrode 401 to the third light-emitting layer (R) 404R tothe desired value ((2m′+1)λ_(R)/4, where m′ is a natural number), thephases of the first reflected light and the first incident light can bealigned with each other and the light emitted from the thirdlight-emitting layer (R) 404R can be amplified.

Note that strictly speaking, the optical path length from the reflectiveelectrode 401 to the third light-emitting layer (R) 404R can be theoptical path length from a reflection region in the reflective electrode401 to a light-emitting region in the third light-emitting layer (R)404R. However, it is difficult to precisely determine the positions ofthe reflection region in the reflective electrode 401 and thelight-emitting region in the third light-emitting layer (R) 404R;therefore, it is presumed that the above effect can be sufficientlyobtained wherever the reflection region and the light-emitting regionmay be set in the reflective electrode 401 and the third light-emittinglayer (R) 404R, respectively.

Next, in the second light-emitting element (G) 410G, the optical pathlength from the reflective electrode 401 to the second light-emittinglayer (G) 404G is adjusted to a desired thickness ((2m″+1)λ_(G)/4, wherem″ is a natural number); thus, light emitted from the secondlight-emitting layer (G) 404G can be amplified. Light (second reflectedlight) that is reflected by the reflective electrode 401 of the lightemitted from the second light-emitting layer (G) 404G interferes withlight (second incident light) that directly enters the semi-transmissiveand semi-reflective electrode 402 from the second light-emitting layer(G) 404G Therefore, by adjusting the optical path length from thereflective electrode 401 to the second light-emitting layer (G) 404G tothe desired value ((2m″+1)λ_(G)/4, where m″ is a natural number), thephases of the second reflected light and the second incident light canbe aligned with each other and the light emitted from the secondlight-emitting layer (G) 404G can be amplified.

Note that strictly speaking, the optical path length from the reflectiveelectrode 401 to the second light-emitting layer (G) 404G can be theoptical path length from a reflection region in the reflective electrode401 to a light-emitting region in the second light-emitting layer (G)404G. However, it is difficult to precisely determine the positions ofthe reflection region in the reflective electrode 401 and thelight-emitting region in the second light-emitting layer (G) 404G;therefore, it is presumed that the above effect can be sufficientlyobtained wherever the reflection region and the light-emitting regionmay be set in the reflective electrode 401 and the second light-emittinglayer (G) 404G, respectively.

Next, in the third light-emitting element (B) 410B, the optical pathlength from the reflective electrode 401 to the first light-emittinglayer (B) 404B is adjusted to a desired thickness ((2m′″+1)λ_(B)/4,where m′″ is a natural number); thus, light emitted from the firstlight-emitting layer (B) 404B can be amplified. Light (third reflectedlight) that is reflected by the reflective electrode 401 of the lightemitted from the first light-emitting layer (B) 404B interferes withlight (third incident light) that directly enters the semi-transmissiveand semi-reflective electrode 402 from the first light-emitting layer(B) 404B. Therefore, by adjusting the optical path length from thereflective electrode 401 to the first light-emitting layer (B) 404B tothe desired value ((2m′″+1)λ_(B)/4, where m′″ is a natural number), thephases of the third reflected light and the third incident light can bealigned with each other and the light emitted from the firstlight-emitting layer (B) 404B can be amplified.

Note that strictly speaking, the optical path length from the reflectiveelectrode 401 to the first light-emitting layer (B) 404B in the thirdlight-emitting element can be the optical path length from a reflectionregion in the reflective electrode 401 to a light-emitting region in thefirst light-emitting layer (B) 404B. However, it is difficult toprecisely determine the positions of the reflection region in thereflective electrode 401 and the light-emitting region in the firstlight-emitting layer (B) 404B; therefore, it is presumed that the aboveeffect can be sufficiently obtained wherever the reflection region andthe light-emitting region may be set in the reflective electrode 401 andthe first light-emitting layer (B) 404B, respectively.

Note that although each of the light-emitting elements in theabove-described structure includes a plurality of light-emitting layersin the EL layer, the present invention is not limited thereto; forexample, the structure of the tandem light-emitting element which isdescribed in Embodiment 3 can be combined, in which case a plurality ofEL layers and a charge generation layer interposed therebetween areprovided in one light-emitting element and one or more light-emittinglayers are formed in each of the EL layers.

The light-emitting device described in this embodiment has a microcavitystructure, in which light with wavelengths which differ depending on thelight-emitting elements can be extracted even when they include the sameEL layer, so that it is not needed to form light-emitting elements forthe colors of R, G, and B. Therefore, the above structure isadvantageous for full color display owing to easiness in achievinghigher resolution display or the like. Note that a combination withcoloring layers (color filters) is also possible. In addition, emissionintensity with a predetermined wavelength in the front direction can beincreased, whereby power consumption can be reduced. The above structureis particularly useful in the case of being applied to a color display(image display device) including pixels of three or more colors but mayalso be applied to lighting or the like.

Embodiment 5

In this embodiment, a light-emitting device including a light-emittingelement which is one embodiment of the present invention will bedescribed.

The light-emitting device can be either a passive matrix light-emittingdevice or an active matrix light-emitting device. Note that any of thelight-emitting elements described in the other embodiments can beapplied to the light-emitting device described in this embodiment.

In this embodiment, an active matrix light-emitting device is describedwith reference to FIGS. 8A and 8B.

Note that FIG. 8A is a top view illustrating a light-emitting device andFIG. 8B is a cross-sectional view taken along the chain line A-A′ inFIG. 8A. The active matrix light-emitting device according to thisembodiment includes a pixel portion 502 provided over an elementsubstrate 501, a driver circuit portion (a source line driver circuit)503, and driver circuit portions (gate line driver circuits) 504 (504 aand 504 b). The pixel portion 502, the driver circuit portion 503, andthe driver circuit portions 504 are sealed between the element substrate501 and a sealing substrate 506 with a sealant 505.

In addition, there is provided a lead wiring 507 over the elementsubstrate 501. The lead wiring 507 is provided for connecting anexternal input terminal through which a signal (e.g., a video signal, aclock signal, a start signal, or a reset signal) or a potential from theoutside is transmitted to the driver circuit portion 503 and the drivercircuit portions 504. Here is shown an example in which a flexibleprinted circuit (FPC) 508 is provided as the external input terminal.Although only the FPC is illustrated, this FPC may be provided with aprinted wiring board (PWB). The light-emitting device in thisspecification includes, in its category, not only the light-emittingdevice itself but also the light-emitting device provided with the FPCor the PWB.

Next, a cross-sectional structure is described with reference to FIG.8B. The driver circuit portions and the pixel portion are formed overthe element substrate 501; here are illustrated the driver circuitportion 503 which is the source line driver circuit and the pixelportion 502.

The driver circuit portion 503 is an example where a CMOS circuit isformed, which is a combination of an n-channel TFT 509 and a p-channelTFT 510. Note that a circuit included in the driver circuit portion maybe formed using any of various circuits, such as a CMOS circuit, a PMOScircuit, or an NMOS circuit. Although a driver-integrated type in whicha driver circuit is formed over the substrate is described in thisembodiment, the present invention is not limited to this type, and thedriver circuit can be formed outside the substrate.

The pixel portion 502 includes a plurality of pixels each of whichincludes a switching TFT 511, a current control TFT 512, and a firstelectrode (anode) 513 which is electrically connected to a wiring (asource electrode or a drain electrode) of the current control TFT 512.Note that an insulator 514 is formed to cover end portions of the firstelectrode (anode) 513. In this embodiment, the insulator 514 is formedusing a positive photosensitive acrylic resin.

The insulator 514 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof in order to obtainfavorable coverage by a film which is to be stacked over the insulator514. For example, in the case of using a positive photosensitive acrylicresin as a material for the insulator 514, the insulator 514 preferablyhas a curved surface with a curvature radius (0.2 μm to 3 μm) at theupper end portion. The insulator 514 can be formed using either anegative photosensitive resin or a positive photosensitive resin. It ispossible to use, without limitation to an organic compound, either anorganic compound or an inorganic compound such as silicon oxide orsilicon oxynitride.

An EL layer 515 and a second electrode (cathode) 516 are stacked overthe first electrode (anode) 513. In the EL layer 515, at least alight-emitting layer is provided. The light-emitting layer has such astacked-layer structure as described in Embodiment 1. Further, in the ELlayer 515, a hole-injection layer, a hole-transport layer, anelectron-transport layer, an electron-injection layer, a chargegeneration layer, and the like can be provided as appropriate inaddition to the light-emitting layer.

The stacked-layer structure including the first electrode (anode) 513,the EL layer 515, and the second electrode (cathode) 516 forms alight-emitting element 517. For the first electrode (anode) 513, the ELlayer 515, and the second electrode (cathode) 516, the materialsdescribed in Embodiment 2 can be used. Although not illustrated, thesecond electrode (cathode) 516 is electrically connected to the FPC 508which is an external input terminal.

Although the cross-sectional view in FIG. 8B illustrates only onelight-emitting element 517, a plurality of light-emitting elements isarranged in a matrix in the pixel portion 502. Light-emitting elementswhich provide three kinds of light emission (R, G, and B) areselectively formed in the pixel portion 502, whereby a light-emittingdevice capable of full color display can be fabricated. Alternatively, alight-emitting device capable of full color display may be fabricated bya combination with coloring layers (color filters).

Further, the sealing substrate 506 is attached to the element substrate501 with the sealant 505, whereby the light-emitting element 517 isprovided in a space 518 surrounded by the element substrate 501, thesealing substrate 506, and the sealant 505. The space 518 may be filledwith an inert gas (such as nitrogen or argon) or the sealant 505.

An epoxy-based resin, low-melting-point glass, or the like is preferablyused for the sealant 505. It is preferable that such a material do nottransmit moisture or oxygen as much as possible. As the sealingsubstrate 506, a glass substrate, a quartz substrate, or a plasticsubstrate formed of fiberglass reinforced plastic (FRP), polyvinylfluoride) (PVF), polyester, acrylic, or the like can be used.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, examples of a variety of electronic devices whichare completed using a light-emitting device will be described withreference to FIGS. 9A to 9D and FIGS. 10A to 10C. The light-emittingdevice is fabricated using a light-emitting element which is oneembodiment of the present invention.

Examples of electronic devices to which the light-emitting device isapplied are television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, cameras such as digitalcameras and digital video cameras, digital photo frames, cellular phones(also referred to as portable telephone devices), portable gamemachines, portable information terminals, audio playback devices, largegame machines such as pin-ball machines, and the like. Specific examplesof these electronic devices are illustrated in FIGS. 9A to 9D.

FIG. 9A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.The display portion 7103 is capable of displaying images, and alight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated with an operation switchprovided in the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. Furthermore, the remote controller 7110 may be providedwith a display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, general television broadcastingcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 9B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using a light-emitting device for thedisplay portion 7203.

FIG. 9C illustrates a portable game machine, which includes twohousings, i.e., a housing 7301 and a housing 7302, connected to eachother via a joint portion 7303 so that the portable game machine can beopened or closed. A display portion 7304 is incorporated in the housing7301 and a display portion 7305 is incorporated in the housing 7302. Inaddition, the portable game machine illustrated in FIG. 9C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), and a microphone 7312),and the like. It is needless to say that the structure of the portablegame machine is not limited to the above structure as long as alight-emitting device is used for at least either the display portion7304 or the display portion 7305, or both, and may include otheraccessories as appropriate. The portable game machine illustrated inFIG. 9C has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. Note that the portable game machine illustrated in FIG.9C can have a variety of functions without limitation to those above.

FIG. 9D illustrates an example of a cellular phone. A cellular phone7400 is provided with a display portion 7402 incorporated in a housing7401, operation buttons 7403, an external connection port 7404, aspeaker 7405, a microphone 7406, and the like. Note that the cellularphone 7400 is manufactured using a light-emitting device for the displayportion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 9D is touched with a finger or the like, data can be input to thecellular phone 7400. Further, operations such as making a call andcreating e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes for the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are mixed.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 7400, display on the screen of the display portion 7402can be automatically changed by determining the orientation of thecellular phone 7400 (whether the cellular phone is placed horizontallyor vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401.Alternatively, the screen modes can be changed depending on the kind ofimage displayed on the display portion 7402. For example, when a signalfor an image to be displayed on the display portion is data of movingimages, the screen mode is changed to the display mode. When the signalis text data, the screen mode is changed to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal identification can be performed. Furthermore, when a backlightor a sensing light source which emits near-infrared light is providedfor the display portion, an image of a finger vein, a palm vein, or thelike can also be taken.

FIGS. 10A and 10B illustrate a foldable tablet terminal. The tabletterminal is opened in FIG. 10A. The tablet terminal includes a housing9630, a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038. The tablet terminal is manufacturedusing the light-emitting device for either the display portion 9631 a orthe display portion 9631 b or both.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9637 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display keyboard buttons in the whole region to be a touch panel,and the display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power saver switch 9036 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. In addition to the optical sensor, another detectiondevice including a sensor for detecting inclination, such as a gyroscopeor an acceleration sensor, may be incorporated in the tablet terminal.

FIG. 10A shows an example in which the display portion 9631 a and thedisplay portion 9631 b have the same display area; however, withoutlimitation thereon, one of the display portions may be different fromthe other display portion in size and display quality. For example, onedisplay panel may be capable of higher-definition display than the otherdisplay panel.

The tablet terminal is closed in FIG. 10B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. In FIG. 10B, astructure including the battery 9635 and the DCDC converter 9636 isillustrated as an example of the charge and discharge control circuit9634.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 10A and 10B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 can beprovided on one or both surfaces of the housing 9630 and the battery9635 can be charged efficiently. The use of a lithium ion battery as thebattery 9635 is advantageous in downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 10B will be described with reference toa block diagram in FIG. 10C. The solar cell 9633, the battery 9635, theDCDC converter 9636, a converter 9638, switches SW1 to SW3, and adisplay portion 9631 are illustrated in FIG. 10C, and the battery 9635,the DCDC converter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 10B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell 9633 is stepped up or down by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is stepped up or down by the converter 9638 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and the switch SW2 is turned on so that the battery9635 may be charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, a non-contact electric power transmission modulewhich transmits and receives power wirelessly (without contact) tocharge the battery 9635, or a combination of the solar cell 9633 andanother means for charge may be used.

It is needless to say that an embodiment of the present invention is notlimited to the electronic device illustrated in FIGS. 10A to 10C as longas the display portion described in the above embodiment is included.

As described above, the electronic devices can be obtained by the use ofthe light-emitting device which is one embodiment of the presentinvention. The light-emitting device has a remarkably wide applicationrange, and can be applied to electronic devices in a variety of fields.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 7

In this embodiment, examples of lighting devices will be described withreference to FIG. 11. A light-emitting device including a light-emittingelement which is one embodiment of the present invention is applied tothe lighting devices.

FIG. 11 illustrates an example in which a light-emitting device is usedfor an interior lighting device 8001. Since the light-emitting devicecan have a larger area, a lighting device having a large area can alsobe formed. In addition, a lighting device 8002 in which a light-emittingregion has a curved surface can also be formed with the use of a housingwith a curved surface. A light-emitting element included in thelight-emitting device described in this embodiment is in the form of athin film, which allows the housing to be designed more freely.Therefore, the lighting device can be elaborately designed in a varietyof ways. Further, a wall of the room may be provided with a large-sizedlighting device 8003.

Moreover, when the light-emitting device is used for a table by beingused as a surface of a table, a lighting device 8004 which has afunction as a table can be obtained. When the light-emitting device isused as part of other furniture, a lighting device which has a functionas the furniture can be obtained.

In this manner, a variety of lighting devices to which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example 1

In this example, a light-emitting element 1 which is one embodiment ofthe present invention is described with reference to FIG. 12. Chemicalformulae of materials used in this example are shown below.

((Fabrication of Light-Emitting Element 1))

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. Note that thethickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 1 over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder in the vacuum evaporationapparatus so that a surface on which the first electrode 1101 wasprovided faced downward. In this example, a case is described in which ahole-injection layer 1111, a hole-transport layer 1112, a light-emittinglayer 1113, an electron-transport layer 1114, and an electron-injectionlayer 1115 which are included in an EL layer 1102 are sequentiallyformed by a vacuum evaporation method.

The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. Then, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation:DBT3P-II) and molybdenum(VI) oxide were co-evaporated with a mass ratioof DBT3P-II (abbreviation) to molybdenum oxide being 4:2, whereby thehole-injection layer 1111 was formed over the first electrode 1101. Thethickness of the hole-injection layer 1111 was set to 20 nm. Note thatco-evaporation is an evaporation method by which a plurality ofdifferent substances is concurrently vaporized from respective differentevaporation sources.

Next, the hole-transport layer 1112 was formed by evaporation of4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)to a thickness of 20 nm.

Next, the light-emitting layer 1113 was formed over the hole-transportlayer 1112. The light-emitting layer 1113 having a stacked-layerstructure was formed by forming a first light-emitting layer 1113 a witha thickness of 20 nm by co-evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) with a mass ratio of 2mDBTBPDBq-II(abbreviation) to PCBBiF (abbreviation) and [Ir(tBuppm)₂(acac)](abbreviation) being 0.7:0.3:0.05, and then forming a secondlight-emitting layer 1113 b with a thickness of 20 nm by co-evaporationof 2mDBTBPDBq-II (abbreviation),N,N′-bis(9,9-dimethylfluoren-2-yl)-N,N′-di(biphenyl-4-yl)-1,4-phenylenediamine(abbreviation: FBi2P), andbis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(divm)]) with a mass ratio of2mDBTBPDBq-II (abbreviation) to FBi2P (abbreviation) and[Ir(dmdppr-dmp)₂(divm)] (abbreviation) being 0.8:0.2:0.05.

Next, over the light-emitting layer 1113, the electron-transport layer1114 was formed in such a manner that a film of 2mDBTBPDBq-II(abbreviation) was formed by evaporation to a thickness of 15 nm andthen a film of bathophenanthroline (abbreviation: Bphen) was formed byevaporation to a thickness of 10 nm. Further, over theelectron-transport layer 1114, a film of lithium fluoride was formed byevaporation to a thickness of 1 nm to form the electron-injection layer1115.

Lastly, over the electron-injection layer 1115, an aluminum film wasformed by evaporation to a thickness of 200 nm as a second electrode1103 functioning as a cathode. Thus, the light-emitting element 1 wasfabricated. Note that in all the above evaporation steps, evaporationwas performed by a resistance-heating method.

Table 1 shows an element structure of the light-emitting element 1obtained as described above.

TABLE 1 First Hole-injection Hole-transport Light-emitting Electron-Electron- Second Electrode Layer Layer Layer transport Layer injectionLayer Electrode Light- ITSO DBT3P-II: BPAFLP * ** 2mDBTBPDBq- Bphen LiFAl emitting (110 nm) MoO_(x) (20 nm) II (10 nm) (1 nm) (200 nm) Element1 (4:2 20 nm) (15 nm) *2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)](0.7:0.3:0.05 20 nm) **2mDBTBPDBq-II:FBi2P:[Ir(dmdppr-dmp)₂(divm)](0.8:0.2:0.05 20 nm)

The fabricated light-emitting element 1 was sealed in a glove boxcontaining a nitrogen atmosphere so as not to be exposed to air(specifically, a sealant was applied to an outer edge of the element andheat treatment was performed at 80° C. for 1 hour at the time ofsealing).

((Operation Characteristics of Light-emitting Element 1))

Operation characteristics of the fabricated light-emitting element 1were measured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

First, FIG. 13 shows current density-luminance characteristics of thelight-emitting element 1. In FIG. 13, the vertical axis representsluminance (cd/m²), and the horizontal axis represents current density(mA/cm²). FIG. 14 shows voltage-luminance characteristics of thelight-emitting element 1. In FIG. 14, the vertical axis representsluminance (cd/m²), and the horizontal axis represents voltage (V). FIG.15 shows luminance-current efficiency characteristics of thelight-emitting element 1. In FIG. 15, the vertical axis representscurrent efficiency (cd/A), and the horizontal axis represents luminance(cd/m²). FIG. 16 shows voltage-current characteristics of thelight-emitting element 1. In FIG. 16, the vertical axis representscurrent (mA), and the horizontal axis represents voltage (V).

FIG. 15 reveals high efficiency of the light-emitting element 1 that isone embodiment of the present invention. Table 2 below shows initialvalues of main characteristics of the light-emitting element 1 at aluminance of about 1000 cd/m².

TABLE 2 External Current Chroma- Lumi- Current Power Quantum VoltageCurrent Density ticity nance Efficiency Efficiency Efficiency (V) (mA)(mA/cm²) (x, y) (cd/m² ) (cd/A) (lm/W) (%) Light-emitting 3 0.064 1.6(0.51, 1000 62 64 22 Element 1 0.48)

The above results show that the light-emitting element 1 fabricated inthis example has high external quantum efficiency, which means its highemission efficiency.

FIG. 17 shows an emission spectrum when a current at a current densityof 25 mA/cm² was supplied to the light-emitting element 1. As shown inFIG. 17, the emission spectrum of the light-emitting element 1 has twopeaks at around 546 nm and 615 nm, which suggests that the two peaks arerespectively derived from emission from the phosphorescentorganometallic iridium complexes [Ir(tBuppm)₂(acac)] and[Ir(dmdppr-dmp)₂(divm)].

Note that by cyclic voltammetry, the HOMO level of PCBBiF, which is thesecond organic compound with a hole-transport property, was estimated tobe −5.36 eV, and the HOMO level of FBi2P, which is the third organiccompound with a hole-transport property, was estimated to be −5.13 eV.Thus, the second organic compound has a lower HOMO level than the thirdorganic compound.

Here, FIG. 18 shows an emission spectrum of a thin film of2mDBTBPDBq-II, which is the first organic compound with anelectron-transport property, an emission spectrum of a thin film ofPCBBiF, which is the second organic compound with a hole-transportproperty, and an emission spectrum of a mixed film (co-evaporation film)of 2mDBTBPDBq-II and PCBBiF. Further, FIG. 19 shows an emission spectrumof a thin film of 2mDBTBPDBq-II, which is the first organic compoundwith an electron-transport property, an emission spectrum of a thin filmof FBi2P, which is the third organic compound with a hole-transportproperty, and an emission spectrum of a mixed film (co-evaporation film)of 2mDBTBPDBq-II and FBi2P.

In each of FIG. 18 and FIG. 19, the emission wavelength of the mixedfilm is located on a longer wavelength side with respect to the emissionwavelengths of the single films of the materials, which indicates thatthe first organic compound (2mDBTBPDBq-II) and the second organiccompound (PCBBiF) formed an exciplex and the first organic compound(2mDBTBPDBq-II) and the third organic compound (FBi2P) formed anexciplex. In addition, while the exciplex formed by the first organiccompound (2mDBTBPDBq-II) and the third organic compound (FBi2P) has anemission wavelength of 553 nm, the exciplex formed by the first organiccompound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) has anemission wavelength of 511 nm to show that it has higher energy.

This application is based on Japanese Patent Application serial no.2012-091533 filed with Japan Patent Office on Apr. 13, 2012, JapanesePatent Application serial no. 2012-225061 filed with Japan Patent Officeon Oct. 10, 2012, and Japanese Patent Application serial no. 2013-047830filed with Japan Patent Office on Mar. 11, 2013, the entire contents ofwhich are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: an anode anda cathode; a first light-emitting layer over the anode, the firstlight-emitting layer comprising a first light-emitting substance, afirst organic compound, and a second organic compound; and a secondlight-emitting layer over the first light-emitting layer, the secondlight-emitting layer comprising a second light-emitting substance, thefirst organic compound, and a third organic compound, wherein a HOMOlevel of the second organic compound is lower than a HOMO level of thethird organic compound, wherein the first organic compound and thesecond organic compound form an exciplex, and wherein an energy of theexciplex is transferred from a singlet state to the first light-emittingsubstance.
 2. The light-emitting element according to claim 1, whereinan emission spectrum of the exciplex overlaps with an absorption bandhaving the longest wavelength of the first light-emitting substance. 3.The light-emitting element according to claim 1, wherein the firstlight-emitting layer is in contact with the second light-emitting layer.4. The light-emitting element according to claim 1, wherein the firstlight-emitting substance is a phosphorescent compound or a thermallyactivated delayed fluorescence material.
 5. The light-emitting elementaccording to claim 1, wherein the second light-emitting substance is aphosphorescent compound or a thermally activated delayed fluorescencematerial.
 6. A light-emitting device comprising: a transistor over asubstrate; and the light-emitting element according to claim 1 over thetransistor.
 7. A light-emitting element comprising: an anode and acathode; a first light-emitting layer over the anode, the firstlight-emitting layer comprising a first light-emitting substance, afirst organic compound, and a second organic compound; and a secondlight-emitting layer over the first light-emitting layer, the secondlight-emitting layer comprising a second light-emitting substance, thefirst organic compound, and a third organic compound, wherein a HOMOlevel of the second organic compound is lower than a HOMO level of thethird organic compound, wherein the first organic compound and the thirdorganic compound form an exciplex, and wherein an energy of the exciplexis transferred from a singlet state to the second light-emittingsubstance.
 8. The light-emitting element according to claim 7, whereinan emission spectrum of the exciplex overlaps with an absorption bandhaving the longest wavelength of the second light-emitting substance. 9.The light-emitting element according to claim 7, wherein the firstlight-emitting layer is in contact with the second light-emitting layer.10. The light-emitting element according to claim 7, wherein the firstlight-emitting substance is a phosphorescent compound or a thermallyactivated delayed fluorescence material.
 11. The light-emitting elementaccording to claim 7, wherein the second light-emitting substance is aphosphorescent compound or a thermally activated delayed fluorescencematerial.
 12. A light-emitting device comprising: a transistor over asubstrate; and the light-emitting element according to claim 7 over thetransistor.
 13. A light-emitting element comprising: an anode and acathode; a first light-emitting layer over the anode, the firstlight-emitting layer comprising a first light-emitting substance and afirst organic compound; and a second light-emitting layer over the firstlight-emitting layer, the second light-emitting layer comprising asecond light-emitting substance, the first organic compound, and asecond organic compound, wherein a LUMO level of the first organiccompound is lower than a LUMO level of the second organic compound, andwherein the first organic compound and the second organic compound forman exciplex.
 14. The light-emitting element according to claim 13,wherein an emission spectrum of the exciplex overlaps with an absorptionband having the longest wavelength of the second light-emittingsubstance.
 15. The light-emitting element according to claim 13, whereinthe first light-emitting layer is in contact with the secondlight-emitting layer.
 16. The light-emitting element according to claim13, wherein the first light-emitting substance is a phosphorescentcompound or a thermally activated delayed fluorescence material.
 17. Thelight-emitting element according to claim 13, wherein the secondlight-emitting substance is a phosphorescent compound or a thermallyactivated delayed fluorescence material.
 18. A light-emitting devicecomprising: a transistor over a substrate; and the light-emittingelement according to claim 13 over the transistor.
 19. A light-emittingelement comprising: an anode and a cathode; a first light-emitting layerover the anode, the first light-emitting layer comprising a firstlight-emitting substance, a first organic compound, and a second organiccompound; and a second light-emitting layer over the firstlight-emitting layer, the second light-emitting layer comprising asecond light-emitting substance, the first organic compound, and a thirdorganic compound, wherein a HOMO level of the second organic compound islower than a HOMO level of the third organic compound, wherein the firstorganic compound and the second organic compound form a first exciplex,wherein the first organic compound and the third organic compound form asecond exciplex, wherein an energy of the first exciplex is transferredfrom a singlet state to the first light-emitting substance, and whereinan energy of the second exciplex is transferred from a singlet state tothe second light-emitting substance.
 20. The light-emitting elementaccording to claim 19, wherein an emission spectrum of the firstexciplex overlaps with an absorption band having the longest wavelengthof the first light-emitting substance.
 21. The light-emitting elementaccording to claim 19, wherein the first light-emitting layer is incontact with the second light-emitting layer.
 22. The light-emittingelement according to claim 19, wherein the first light-emittingsubstance is a phosphorescent compound or a thermally activated delayedfluorescence material.
 23. The light-emitting element according to claim19, wherein the second light-emitting substance is a phosphorescentcompound or a thermally activated delayed fluorescence material.
 24. Alight-emitting device comprising: a transistor over a substrate; and thelight-emitting element according to claim 19 over the transistor.